Form, function and evolutionary significance of stridulatory organs in ant nest beetles (Coleoptera: Carabidae: Paussini)

Stridulatory organs in the myrmecophilous carabid beetle tribe Paussini have long been recognized and used as a defining character of some genera and higher level taxa, however their morphology has only roughly been described. Here, we describe the fine morphology of Paussini stridulatory organs using scanning electron (SEM) and focused ion beam (FIB) microscopy. Within this tribe, there are three types of stridulatory organs, each with different positions of the scraper (plectrum) and file (pars stridens). Type I (abdomen-femur type) is located on the abdomen (scraper) and metafemur (file) in the subtribe Paussina (sensu Geiselhardt et al., 2007, Naturwissenschaften 94: 871-894). Type II (thorax-femur type) and Type III (femur-thorax type) are located on the mesothorax and mesofemur in two different genera of the subtribe Platyrhopalina, however in Euplatyrhopalus the scraper is located on the mesothorax and the file is located on the mesofemur (Type II), whereas in the genus Platyrhopalopsis the structures are reversed in that the file is located on the mesothorax and the scaper is located on the mesofemur (Type III). The independent evolution of three types of stridulatory organs in three lineages of Paussini suggests that acoustical communication has played an important role in the evolution of ant nest beetles. While the roles of stridulation in this group remain speculative, we verified that all three types of stridulatory organs are present in both sexes and are similar to stridulatory organs known in their host ants which also use stridulation as a method of communication. We discuss the possibility that the beetles' stridulation could (1) facilitate their exploitation of ant colonies, and (2) be involved in mate recognition and courtship.